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We have developed three distinct luminogenic substrates for measuring the enzyme activity of CYP3A4, the principal drug-metabolizing cytochrome P450 (CYP) in the liver and small intestine. The substrates are luciferin derivatives used with the luciferase based P450-Glo™ technology. Factors that influence the choice of substrate include CYP enzyme selectivity, DMSO sensitivity and source of CYP3A4 (microsomes vs. cells). The selectivity of each substrate for CYP enzymes is demonstrated in assays with recombinant CYPs. These microsome assays can utilize any of the three substrates for detecting inhibition and measuring IC50s. Assays of CYP3A4 from cultured cells use the substrates in a non-lytic approach that detects basal and induced CYP3A enzymeactivity in freshly isolated and cryopreserved hepatocytes and DPX-2 cells (a stable cell-line over-expressing the PXR nuclear receptor). The cell-based assays can be multiplexed with a cell viability assay to measure cytotoxicity of test compounds and for normalization of CYP activity measurements to cell number. These cell- and microsome-based assays are homogeneous and easily configured in high throughput multiwell formats to measure CYP gene induction or enzyme inhibition by new chemical entities.

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